Thanks for clarifying. I was wondering if it was some alternative scenario due to the lower capacity (and, worth noting, ridership exceeding capacity) on the SRT segment, but this confirms that this is the Transit City concept.
The point of peak demand being eastbound departing Oakwood is what I was familiar with, so no surprise there IMO.
The answer seems to be that 5,400 is the number that was developed by the TTC as part of the original Transit City planning (Perhaps in the ECLRT EA? I can't be bothered to look it up.). The number has been consistently reused since (just Google it, people reference it all the time (including you

).
So even though later modelling was done which provided different numbers, including the 6,500 referenced above, communications materials have stuck with the 5,400 number for consistency.
Since the line is considered to be justified at 5,400, they have no reason to promote the higher numbers. That way, when the line opens and the 5,400 estimate is exceeded well before 2031 they can point to how successful the line is. Under promise and over deliver.